Method of calibrating the air flow of a swirler of a gas turbine burner

ABSTRACT

A method of calibrating a swirler for a burner of a turbine engine, the swirler having a plurality of vanes and a plurality of mixing channels between the vanes, wherein each mixing channel directs air from a radially outer end of the mixing channel to a radially inner end of the mixing channel, the method of calibrating the swirler including: determining a flow characteristic of the swirler, calculating the difference between the determined flow characteristic of the swirler and a predetermined flow characteristic of the swirler, and dependent on the difference, applying a calibration to the swirler to alter its flow characteristic, the calibration having a known influence on the flow characteristic such that the altered flow characteristic is within an acceptable tolerance of the predetermined flow characteristic.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2014/076527 filed Dec. 4, 2014, and claims the benefitthereof. The International Application claims the benefit of EuropeanApplication No. EP13197795 filed Dec. 17, 2013. All of the applicationsare incorporated by reference herein in their entirety.

FIELD OF INVENTION

The present invention is related to a calibration arrangement for aswirler of a burner of a gas turbine, the swirler comprising a pluralityof vanes and a plurality of mixing channels between the vanes, whereineach mixing channel is enabled to channel air from a radially outer endof the mixing channel to a radially inner end of the mixing channel.Further, the invention is related to a swirler for a burner of a gasturbine, comprising calibration arrangement, a plurality of vanes and aplurality of mixing channels between the vanes, wherein each mixingchannel is enabled to channel air from a radially outer end of themixing channel to a radially inner end of the mixing channel, further toa burner of a gas turbine, comprising an air supply, a fuel supply, aswirler and a combustion chamber and further to a gas turbine,comprising at least one burner.

BACKGROUND OF INVENTION

Modern gas turbines are commonly used in industrial applications. Toachieve the goal of an environmental friendly operation of the gasturbine, the gas turbine is operated in a DLE combustion mode (DLE: DryLow Emission) producing low emissions, especially low NOx emissions. Toachieve this goal, a good and uniform mixing of air and fuel in a burnerof the gas turbine has to be achieved. In modern gas turbines swirlersare used for this task.

FIG. 1 shows a sectional view of an example of a gas turbine 40. The gasturbine 40 comprises an air inlet 41, a compressor section 42, a burnersection 44 and a turbine section 45 which are generally arranged in flowseries and generally in the direction of a longitudinal rotation axis81. The gas turbine 40 further comprises a shaft 47 which is rotatableabout rotational axis 81 and which extends longitudinally through thegas turbine 40. The shaft 47 drivingly connects the turbine section 45to the compressor section 42.

In operation of the gas turbine 40, air 80, which is taken in throughthe air inlet 41, is compressed by compressor blades 43 in thecompressor section 42 and delivered to the burner section 44. The burnersection 44 comprises a combustion chamber 63, defined by a double wallcan, and at least one burner 60 fixed to the combustion chamber 63. Thecompressed air 80 passing through the compressor section 42 enters viaan air supply 61 into a swirler 20 and is discharged from the swirler 20into the combustion chamber 63. In mixing channels 22 (not shown) of theswirler 20, the air 80 is mixed with gaseous or liquid fuel, provided bya fuel supply 62 of the burner 60. The air/fuel mixture is burnedafterwards in the combustion chamber 63 and the combustion gas orworking gas from the combustion is channelled to the turbine section 45.

The turbine section 45 comprises a number of turbine blades 46 carryingdiscs attached to the shaft 47. In the present example, two discs eachcarry an annular array of the turbine blades 46 are shown. However, thenumber of blade carrying discs could be different, for instance only onedisc or more than two discs. In addition, guiding vanes 48, which arefixed to a stator of the gas turbine 40, are disposed between theturbine blades 46. The combustion gas from the combustion chamber 63enters the turbine section 45 and drives the turbine blades 46 which inreturn rotates the shaft 47. The guiding vanes 48 serve to optimise theangle of the combustion exhaust gas on the turbine blades 46.

As mentioned above, variations in air/fuel distributions in the burnerhave a negative influence on the temperature distribution and theuniformity of the flame in this specific burner. The variations inair/fuel distributions are mostly caused by the used swirler of theburner. Therefore it is known, to specifically choose the swirler whichis suited best for a specific burner form a plurality of ready builtswirlers held in stock. By doing so, a uniform flame distribution in theburner can be achieved. This leads to a low emission operation of thegas turbine and in addition to a longer life time of the hot componentsof the gas turbine.

SUMMARY OF INVENTION

It is an object of the present invention to solve the aforesaid problemsand drawbacks at least partly. In particular, it is an object of thepresent invention to provide calibration means, a swirler, a burner anda gas turbine, which allow a low emission operation of a gas turbine andimprove the life time of the gas turbine especially in an easy and costefficient way.

The aforesaid problems are solved by calibrations means for a swirlerfor a burner of a gas turbine and by a gas turbine having a calibratedswirler. Further features and details of the present invention resultfrom the subclaims, the description and the drawings. Features anddetails discussed with respect to the calibration means can also beapplied to the swirler, the burner and the gas turbine and vice versa,if of technical sense.

According to the first aspect of the invention the aforesaid object isachieved by a calibration means for a swirler of a burner of a gasturbine, the swirler comprising a plurality of vanes and a plurality ofmixing channels between the vanes, wherein each mixing channels isenable to direct air from a radially outer end of the mixing channel toa radially inner end of the mixing channel. The calibration meansaccording to the invention is characterized in that the calibrationmeans can be arranged at the swirler in such a way that the calibrationmeans is enabled to manipulate the flow of the channelled air in atleast one of the mixing channels.

The swirler described in the preamble is used in a burner of a gasturbine to produce an air/fuel mixture. This air/fuel mixture isafterwards burned in a combustion chamber of the burner. To achieve avery uniform temperature distribution in the flame and therefore tooperate the burner of the gas turbine in a low NOx emission regime andadditionally extending the life time of the hot components of theburner, an initially uniform distribution of the air/fuel mixture isnecessary. A calibration means according to the invention allows to usea ready built swirler in a burner of a gas turbine and to achieve anespecially uniform distribution of the air/fuel mixture provided by theswirler without exchanging the swirler in total. The calibration meansaccording to the invention is able to manipulate the flow of thechannelled air in at least one of the mixing channels. Therefore theflow of the channelled air in this at least one mixing channel of theswirler can be changed such that the swirler in total provides a uniformair/fuel mixture distribution. Especially and in addition thecalibration means can enhance turbulences in the channelled air in theat least one mixing channel of the swirler and therefore enhance themixing of the air with fuel fluid. Thus an even better mixing of air andfuel can be achieved. By using of calibration means according to theinvention therefore a broad variety of swirlers can be adapted to beused in a specific burner of a gas turbine. Only the calibration meanshas to be chosen such that with a specific swirler a uniformdistribution of the air/fuel mixture provided by the swirler can beachieved. This allows reducing the amount of swirlers held in stock andtherefore lowers the cost in the production and assembling of burnersfor gas turbines. In addition the evenness of the distribution of theair/fuel mixture provided by the swirler can be improved and therefore amore uniform flame temperature in the burner of the gas turbine can beachieved. This causes lower NOx emissions and a longer life time of thehot components in the burner and/or the gas turbine.

Further, calibration means according to the invention can becharacterized in that the calibration means can be arranged at and/ornear the radially outer end of the mixing channels. To achieve a goodmixing of air and fuel, fuel outlets can be arranged in the mixingchannels of the swirler. As mentioned above, with a calibration meansaccording to the invention a flow of the channelled air in the mixingchannels of the swirler with an improved evenness and a betterturbulence characteristic respectively can be created. Such an improvedstream of channelled air in the mixing channels mixes better with thefuel provided from the fuel outlets. Therefore a positioning ofcalibration means according to the invention at and/or near the radiallyouter end of the mixing channel allows ensuring all effects mentionedabove in a very easy way, and especially ensures that the calibrationmeans according to the invention is placed before fuel outlets arrangedin the mixing channels.

In a further advanced arrangement of a calibration means according tothe invention, the calibration means is attached to a closing plate,wherein the closing plate is enabled to be arranged at the swirler. Itis known to use closing plates with swirlers. Such closing plates oftenform the upper capping of the swirler. With calibration means attachedto a closing plate it is therefore especially easy to place thecalibration means in such a position relative to the swirler, in whichthe calibration means are able to manipulate the flow of the channelledair in at least one of the mixing channels of the swirler. For acalibration of the swirler, especially to achieve a uniform distributionof the air/fuel mixture provided by the swirler, it is thereforesufficient, to choose a closing plate with appropriate calibration meansto achieve this goal without changing the swirler in total. This reducesthe costs of the calibration process of a swirler for a burner of a gasturbine.

In addition, calibration means according to the invention can becharacterized in that the calibration means comprises a plurality ofcalibration elements; in particular the calibration means comprises acalibration element for each mixing channel, wherein each calibrationelement is enabled to manipulate flow of the channelled air in one ofthe mixing channels. This feature allows the calibration of severalmixing channels at once and only one calibration means. In particular,all of the mixing channels of a swirler can be calibrated with only onecalibration means. This is a very easy and simple way of achieve a veryuniform distribution of the air/fuel mixture provided by the swirler,the swirler calibrated by calibration means according to the invention.

According to a further development of calibration means according to theinvention, the calibration's elements of the calibration means areconstructed identically. A calibration means with identical calibrationelements can be especially produced more easily, because theconstruction process is the same for all of the calibration elements. Avery cost efficient production of the calibration elements of thecalibration means can therefore be achieved.

Alternative, according to another development of the invention, thecalibration elements of the calibration means are adapted for therespective mixing channel. Especially the calibration elements of thecalibration means can be different from each other. The adaption of acalibration element to a respective mixing channel of the swirler allowsespecial good calibration of the flow of the channelled air in thismixing channel. A very uniform distribution of the channelled air in allof the mixing channels manipulated by calibration elements of thecalibration means can therefore be achieved. A burner of a gas turbinewith a swirler with such calibration means can therefore achieve a verygood burning performance, especially is able to run in a very lowemission mode.

According to another development of the invention a calibration meanscomprises a blocking device to block at least partly the flow of thechannelled air in at least one of the mixing channels, wherein inparticular the blocking device comprises at least one aperture, inparticular a hole. Naturally, such a blocking device can be acalibration element according to the invention. With a blocking device,the amount of air in the respective mixing channel can be reduced in acontrolled way. Even further, such a blocking device produces on itsedges turbulences in the flow of the channelled air. This causes abetter mixing of the channelled air with fuel provided in the swirler.Identical or different blocking devices for several or all of the mixingchannels can be provided achieving the advantages already describedabove in respect to the calibration elements. An aperture, especially ahole, enhances the amount of turbulences caused in the channelled air bythe blocking device. A gain in turbulences in the channelled airimproves the ability to mix with fuel provided in the swirler. An evenbetter mixing of fuel and air can therefore be achieved.

In another alternative development of the invention, the calibrationmeans comprises a wire mesh to manipulate the flow of the channelled airin at least one of the mixing channels. Naturally, also such a wire meshcan be a calibration element according to the invention. Such a wiremesh produces turbulences in the channelled air in a very easy way bythe interaction of the wires of the wire mesh with the channelled air. Awire mesh is very low in weight and easy to produce. In addition, a wiremesh is a mass product and therefore low in costs.

According to a further development of the invention, the wire meshcompletely covers the radially outer end of at least one of the mixingchannels. This ensures a very efficient turbulence production in thechannelled air caused by the wire mesh. Advantageously all mixingchannels are covered completely with the wire mesh. In particular, thecomplete outer boundary of the swirler is circumferential covered with awire mesh. This is a very easy way to automatically cover all radiallyouter ends of all mixing channels.

According to another development of the invention a calibration meanscan be provided, characterized in that the wire mesh has a uniformgauge. Such a wire mesh is a bulk product and therefore extremely low incost. This allows providing a calibration of a swirler in a very lowcost regime.

Further, according to an alternative development of the invention, thewire mesh has a non-uniform gauge, in particular along a height of thecalibration means. With a non-uniform gauge an even better calibration,especially an individual calibration for each of the mixing channels,can be achieved. It is also possible, that near the fuel outlet athinner gauge of wire is used for the wire mesh, causing more turbulencein this area. Therefore a better mixing of such manipulated air with thefuel provided from a fuel outlet can be achieved. In another possibilityto achieve a non-uniform gauge, also the mesh spacing and/or density canbe altered; especially a mesh with 45° up to 90° can be used.

According to a further development of the invention the wires of thewire mesh comprise turbulence generating elements, in particular thewires of the mesh are constructed as swirling elements. Such swirlerelements enhance further the turbulence production caused by the wiremesh and therefore improve the mixing of the manipulated channelled airwith fuel provided in the swirler. Such swirling elements can forinstance be fins or ribs attached to the wires.

Alternative or additional the wires themselves can be constructed asswirling elements. To achieve this, the wires can for instance bespiral-shank or serrated. Naturally other embodiments of swirlerelements or wires constructed as swirler elements are possible.

According to a second aspect of the invention, the object is solved by aswirler for a burner of a gas turbine, comprising calibration means, aplurality of vanes and a plurality of mixing channels between the vanes,wherein each mixing channel is enabled to channel air from a radiallyouter end of the mixing channel to a radially inner end of the mixingchannel. A swirler according to the invention is characterized in thatthe calibration means is constructed according to the first aspect ofthe invention. The use of such a calibration means provides the sameadvantages, which have been discussed in detail according to thecalibration means according to the first aspect of the invention.

Further, according to a third aspect of the invention, the object issolved by a burner of a gas turbine, comprising an air supply, a fuelsupply, a swirler and a combustion chamber. A burner according to theinvention is characterized in that the swirler is constructed accordingto the second aspect of the invention. The use of such a swirlerprovides the same advantages, which have been discussed in detailaccording to a swirler according to the second aspect of the invention.

In addition, according to a fourth aspect of the invention, the objectis solved by a gas turbine, comprising at least one burner. A gasturbine according to the invention is characterized in that the burneris constructing according to the third aspect of the invention. The useof such a burner provides the same advantages, which have been discussedin detail according to a burner according to the third aspect of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with respect to the accompanyingfigures. The figures show schematically:

FIG. 1 a sectional view of a gas turbine,

FIG. 2 a first embodiment of a swirler with calibration means accordingto the invention,

FIG. 3 a second embodiment of a swirler with calibration means accordingto the invention and

FIGS. 4A-4D show alternatives of the second embodiment.

DETAILED DESCRIPTION OF INVENTION

In FIG. 2, a schematic view of a swirler 20 according to the inventionis shown. The swirler 20 has an axis 27 and comprises a plurality ofvanes 21 extending axially from a base plate 26. Between the vanes 21 aplurality of mixing channels 22 are formed, the mixing channels 22facilitate air from a radially outer end or inlet 24 to a radially innerend or outlet 23. Further a fuel supply 62 is shown, which is used toprovide the fuel to be mixed in the swirler 20 with the channelled air.The mixing channels 22 are capped with a closing plate 25. On thisclosing plate 25 calibration means 1 are attached. In this embodiment ofthe calibration means 1, according to the invention, are constructed asblocking devices 4 with apertures 5, the apertures shaped as holes 5.Although shown as holes 5, in other embodiments, the free edge of anyone of the blocking devices 4 can have a scallop 5′ either alone or incombination with a hole 5. Both the holes 5 and scallops 5′ can bereferred to as cut-outs 5, 5′.

In this embodiment for each of the mixing channels 22 a calibrationelement 3, especially shaped as a blocking device 4 is provided. Airchannelled in the mixing channels 22 is manipulated by the blockingdevices 4. In the flow of the air additional turbulences are producedand therefore the mixing of the channelled air with fuel provided by thefuel supply 62 is improved. To find a swirler 20 which is ideal for aspecific burner 60 (not shown) only the calibration means 1 has to beadapted to achieve a uniform flow of the channelled air through theseveral mixing channels 22. A replacement or exchange of the completeswirler 20 to achieve this goal is not necessary. That's why this is avery cost efficient way to calibrate a swirler 20 of a burner 60.

The calibration element 3 is formed as a series of blocking devices 4,which, in this example, are an annular array of plates extending fromthe closing plate 25. Each plate extends to cover part of the mixingchannel 22.

Alternatively, rather than individual plates 4, a ring 9, shown in partby dashed lines, may extend from the closing plate 25. The ring 9comprises an annular array of cut-out 5, 5′, each cut-out being alignedwith a mixing channel.

The plates 4 or ring 9 and the closing plate 25 are integrally formed asone piece or may be formed by bonding or welding the plates 4 or ring 9and the closing plate 25 to form the calibration means 1. The size ofthe cut-out 5, 5′ or even the number of cut-outs can be designed toachieve a desired influence on the flow characteristic through themixing channels 22.

FIG. 3 shows another embodiment of calibrations means 1 for a swirler20. The calibration means 1 comprises a calibration element 3, in thiscase shaped as a wire mesh 6. The wire mesh 6 covers the radially outerends 24 of the mixing channels 22 along the complete height 2 of thecalibration means 1. The calibration means 1 extends around the completeradially outer surface of the swirler, thus covering all the mixingchannels 22. This ensures a very efficient manipulation of the airchannelled through the mixing channels 22 from the radially outer end 24to the radially inner end 23. The wires 7 of the wire mesh 6 can beequipped with swirling elements 8 such as fins or rips. Anotherpossibility is that the wires 7 themselves are constructed as swirlingelements 8, for instance the wires can be spiral-shanked or serrated.Such swirling elements 8 enhance the production of turbulence in the airchannelled through the mixing channels 22 and therefore the mixing ofair with fuel provided in the swirler 20.

One advantage of the present invention is that the calibration means 1can be selected for its known influence on a flow characteristic of theair passing through the swirler. Thus depending on the flowcharacteristic of each individual swirler a calibration means 1 can beselected and applied to that swirler to ensure that all the swirlers onone engine or all engines are within a tolerance of each other and thuseach swirler or engine set of swirlers have a known and desired air flowand performance. This ensures optimal performance of the combustionsystem and ensures a more even temperature distribution for downstreamcomponents. Furthermore, better fuel/air mixing and more consistent oroptimal fuel/air ratios within each combustor is now achievable. Thuscan help reduce and maintain emissions such as carbon, nitrous andsulphur oxides.

FIG. 3 also shows an alternative wire mesh 6′ which has a height 10which is less than the height 2 of the inlet of the mixing channel 22.Thus the calibration means 1 can be designed and/or selected based onits height 10 and therefore its influence on the flow characteristic ofthe air passing through the mixing channels 22 or swirler 20 as a wholeto ensure all calibrated swirlers 20 have a predetermined flowcharacteristic within an acceptable tolerance.

FIGS. 4A-4D show further alternative embodiments of the wire mesh. FIG.4A shows a higher density mesh 6A compared to the mesh 6 shown in FIG.3. The mesh 6A has similarly spaced crossing sets of wires 11, 12. FIG.4B shows a mesh 6B which has a greater number of wires 11 in a firstdirection and a lesser number of wires 12 in a second direction. Eachset of wires 11, 12 can have a spacing chosen to produce a calibrationmeans having a desired influence on the flow characteristic through themixing channels 22.

FIG. 4C shows a mesh 6C having a different wire gauge or thickness ofone or both sets of wires 11, 12. In this case the wires 11, 12 arethicker than as shown in FIG. 3 and therefore the mesh 6C has a greaterinfluence on the flow characteristic through the mixing channels 22 thanthe mesh 6.

FIG. 4D shows a mesh 6D having non-straight wires 11, 12. One or bothsets of wires 11, 12 can be S-shaped, sinusoidal or even castellated.The amplitude and/or frequency of the non-straight shape can determinethe mesh' s influence on the flow characteristic through the mixingchannels 22.

The sets of wires 11, 12 do not need to be perpendicular to one another,but instead can form apertures in square or other parallelogram shapesand indeed any other shapes as shown in FIG. 4D.

It should be appreciated that the wire mesh type calibration means 1 canbe formed by a combination of any of the various parameters describedabove in relation to FIGS. 3, 4A-4D.

Similarly, the plate type calibration means 1 can have variouscombinations of hole or scallop sizes and/or numbers. Although eachswirler is manufactured to the same nominal dimensions it has been foundthat the manufacturing tolerances cause an undesirable variation in flowcharacteristics between swirlers with the aforementioned associatedproblems. The flow characteristic of the swirler can be the swirler' smass flow rate or the effective flow area of the mixing channels.Therefore, each swirler can be tested or evaluated to check its flowcharacteristic and then a calibration means 1 can be fitted. Thecalibration means 1 is selected from a collection of calibration means 1with different and known influences on the flow characteristic toachieve an engine set of calibrated swirlers having flow characteristicswithin an acceptable tolerance.

The present invention particularly lends itself to a method ofcalibrating a swirler for a burner of a turbine engine comprises thesteps of determining a flow characteristic of the swirler, calculatingthe difference between the determined flow characteristic of the swirlerand a predetermined flow characteristic of the swirler, and dependent onthe difference and applying a calibration means 1 to the swirler toalter its flow characteristic. As mentioned above the calibration means1 has a known influence on the flow characteristic such that the alteredflow characteristic is within an acceptable tolerance of thepredetermined flow characteristic. In one example, the acceptabletolerance band is +/−3sigma, where 1 sigma corresponds to 1%. Sigma isthe standard deviation of components as result of the manufacturingprocess.

Previously, the swirler was manufactured to a nominal configuration andwhere the flow characteristic was below a certain level the swirler wasscrapped. For the present inventive calibration method, the swirler 20is manufactured to a nominal configuration to give a nominal flowcharacteristic which is greater than the desired flow characteristic.Therefore, for some swirlers with a flow characteristic at a low end ofthe tolerance range there may be no need to apply a calibration deviceor a calibration means with a relatively small influence on the flowcharacteristic. For other swirlers with a flow characteristic at a highend of the tolerance range there may be no need to apply a calibrationmeans 1 with a relatively large influence on the flow characteristic. Inone example, the swirler has a nominal flow characteristic which is upto 3sigma greater than the desired flow characteristic.

As mentioned above, the method comprises selecting the calibration means1 from a group of calibration means 1, each calibration means 1 having adifferent and known influence on the flow characteristic of the swirler.Such a group of calibration means 1 comprises calibration means 1 havingknown influences on the flow characteristics of swirlers of 0.5%, 1.0%,1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%. In other words, the calibrationmeans reduces the effective flow area of the mixing channel or at leastthe area of the inlet to the mixing channel. It should be understoodthat the area of the calibration means or it's the open area as apercentage of the area of the mixing channel can be different to itsreduction in the effective flow area of the mixing channel or its massflow rate. Thus selection and application of any one of thesecalibration devices or means will ensure that the swirler is within thedesired tolerance.

Similar to the first embodiment described with reference to FIG. 2, theembodiments of the calibration means 1 in FIGS. 3, 4A-4D may be in theform of a ring 9′ which is applied around the outer perimeter of theswirler vanes 21. Furthermore, the wire meshes 6, 6′, 6A-6D may beformed as a separate ring element or may be formed as part of theclosing plate 25. Alternatively, the calibration means 1 can be weldedor brazed on the base plate 26.

By applying the above inventive method a turbine engine results inhaving an array of combustors with at least one of the combustors havinga calibrated swirler and more particularly all swirlers calibrated.Thus, at least one combustor and advantageously all combustors have aswirler comprising a calibration means 1. Here the turbine engine hasall combustors having their swirlers comprising a flow characteristicwithin the acceptable tolerance of the predetermined flowcharacteristic. This acceptable tolerance is advantageously within 1sigma and can be within 0.5sigma.

It should be noted that the predetermined flow characteristic value canbe set at a level where all swirlers require a calibration means 1. Thisis advantageous because each swirler will benefit from increasedturbulence and improved mixing of the fuel and air by virtue of thecalibration means interacting with the air passing around and/or throughit.

In summary calibration means according to the invention allow acalibration of the channelled air in a swirler in a very easy and costefficient way. Once a swirler is chosen to be used in a burner of a gasturbine calibration means can be used to calibrate the chosen swirlerand to allow a use of the swirler in the optimum location in a gasturbine with an optimum performance. For this calibration for instancecalibration means with different calibration elements such as blockingdevices, wire mesh and/or swirling elements can be used to achieve anoptimum calibration of the swirler. A swirler with such a calibrationmeans has several advantages. A more uniform temperature distribution inthe combustion chamber of the burner using such a swirler with acalibration means can be achieved, thus resulting in a longer life timeof hot components of the burner. In addition, enhanced mixing of fueland air and a reduction of the temperature hot spots result in a loweremission operation of the gas turbine, especially in respect to NOxproduction. Also a reduced down time and an improved serviceability iscaused by the more uniform air distribution in the mixing channelsproduced by the calibration means. In addition during the constructionof the gas turbine an enhanced flexibility in choosing a swirler for thegas turbine and therefore an adequate stock management in respect toswirlers is possible.

1. A method of calibrating a swirler for a burner of a turbine engine,the swirler comprising a plurality of vanes and a plurality of mixingchannels between the vanes, wherein each mixing channel directs air froma radially outer end of the mixing channel to a radially inner end ofthe mixing channel, the method of calibrating the swirler comprising:determining a flow characteristic of the swirler, calculating thedifference between the determined flow characteristic of the swirler anda predetermined flow characteristic of the swirler, and dependent on thedifference, applying a calibration means to the swirler to alter itsflow characteristic, the calibration means having a known influence onthe flow characteristic such that the altered flow characteristic iswithin an acceptable tolerance of the predetermined flow characteristic.2. A method of calibrating a swirler as claimed in claim 1, furthercomprising: forming the swirler having a nominal flow characteristicwhich is greater than the desired flow characteristic.
 3. A method ofcalibrating a swirler as claimed in claim 2 wherein the swirler has anominal flow characteristic which is up to 3sigma greater than thedesired flow characteristic.
 4. A method of calibrating a swirler asclaimed in claim 1, further comprising: selecting the calibration meansfrom a group of calibration means, each calibration means of the grouphaving a different and known influence on the flow characteristic of theswirler.
 5. A method of calibrating a swirler as claimed in claim 4wherein the group of calibration means comprises a number of calibrationmeans having known influences on the flow characteristics of the swirlerof at least two of 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%.
 6. Amethod of calibrating a swirler as claimed in claim 1 wherein thecalibration means comprises a mesh (3, 6, 6′, 6A, 6B, 6C, 6D) having alattice of wires, wherein the method further comprises: selecting anyone or more of the spacing of the wires, the thickness and the shape ofthe wires to determine the known influence on the flow characteristicsof swirler.
 7. A method of calibrating a swirler as claimed in claim 1wherein the calibration means comprises a plate having at least onecut-out wherein the method further comprises: selecting any one or moreof the size and number of the cut-out to determine the known influenceon the flow characteristics of swirler.
 8. A method of calibrating aswirler as claimed in claim 1 wherein the calibration means is in theform of a ring, the method further comprising: applying the calibrationmeans around the outer perimeter of the swirler.
 9. A method ofcalibrating a swirler as claimed in claim 5 wherein the each mixingchannel has a height, the method further comprises: selecting a heightof the calibration means to cover at least a part of the height of themixing channel to at least partly influence the flow characteristics ofswirler.
 10. A method of calibrating a swirler as claimed in claim 1wherein the acceptable tolerance of the predetermined flowcharacteristic is 1 sigma.
 11. A method of calibrating a swirler asclaimed in claim 1 wherein the step of applying a calibration meanscomprises applying the calibration means around an outer perimeter ofthe swirler vanes.
 12. A turbine engine having an array of combustors,each combustor having a swirler, wherein the swirler comprises aplurality of vanes and a plurality of mixing channels between the vanes,wherein each mixing channel directs air from a radially outer end of themixing channel to a radially inner end of the mixing channel, andwherein at least one of the swirlers has been calibrated by: determininga flow characteristic of the swirler, calculating the difference betweenthe determined flow characteristic of the swirler and a predeterminedflow characteristic of the swirler, and dependent on the difference,applying a calibration means to the swirler to alter its flowcharacteristic, the calibration means having a known influence on theflow characteristic such that the altered flow characteristic is withinan acceptable tolerance of the predetermined flow characteristic.
 13. Aturbine engine as claimed in claim 12, wherein at least one of theswirlers comprises a calibration means.
 14. A turbine engine as claimedin claim 13, wherein all of the swirlers comprise a calibration means.15. A turbine engine as claimed in claim 12, wherein all the swirlerscomprise a flow characteristic within the acceptable tolerance of thepredetermined flow characteristic of 1 sigma.